The constitution and general mode of operation of a thermal type ink jet printhead, and in particular of the type known as “top shooter”, i.e. that emits the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the art, and, therefore will not be described in detail here, while only some characteristics of relevance for the purposes of the understanding of this invention will be described more particularly.
With reference to FIG. 1, an integrated printhead (head) 10, according to the known art, is made of an integrated circuit, for instance NMOS or bipolar type and comprises a plurality of nozzles 31, positioned on the head 10 according to a predefined order and suitable for ejecting ink on a medium, generally of paper, and a plurality of inputs or contacts 23; provided for connecting the head 10 to an external control circuit, suitable for commanding the selective actuation of the various nozzles 31.
The known head 10 (FIG. 2) is made of a grid-like driving circuit (M×N) comprising a plurality M of actuating assemblies 14. Each actuating assembly 14 in turn comprises a plurality N of selecting-elements or transistors 12 and an equivalent number of actuating elements or resistors 11 which are provided for causing, in a known way, ejection of the ink from the nozzles 31.
Also described by the U.S. Pat. No. 5,363,134 is an integrated printhead that comprises an encoding circuit, in which, by burning programmable fuses, information can be stored about the general characteristics of the head, such as for instance: colour head, its resolution, number of nozzles.
The encoding circuit is integrated on the same substrate as the selecting and activating circuit of the resistors and is composed of a row of programmable fuses, each of which is connected in series with a transistor.
Besides, each couple comprising a fuse and a transistor of the encoding circuit, is permanently connected to an Address Line and has the advantage of permitting reading of the coding stored without increasing the number of head connections.
It is in fact advantageous to have the possibility of identifying, through the printer's control circuit, a set of head characteristics, stored during the production of the head, that can cause a non-uniformity of operation between the various nozzles, considerably worsening the print quality.
Some of the characteristics that may be stored are, for instance, misalignments or differences of shape, diameter or center distance between the nozzles.
In possession of this information, the control circuit can compensate, by varying the energy supplied to the resistors, any differences in volume of the ink droplets or of speeds which are caused by these non-uniformities.
This solution has the disadvantage, however, of adding an encoding circuit, formed by N couples of fuses and transistors, where N is the number of addresses in the grid, to those already existing on the head, with a relative increase in the surface area of the integrated circuit and greater costs and manufacturing difficulties. In addition, the encoding circuit can contain at most N fuses.